The Astronomical Journal, 125:2964–2974, 2003 June # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A. CORONAGRAPHIC IMAGING OF 3C 273 WITH THE ADVANCED CAMERA FOR SURVEYS1 A. R. Martel,2 H. C. Ford,2 H. D. Tran,2 G. D. Illingworth,3 J. E. Krist,4 R. L. White2,4 W. B. Sparks,4 C. Gronwall,5 N. J. G. Cross,2 G. F. Hartig,4 M. Clampin,4 D. R. Ardila,2 F. Bartko,6 N. Benı´tez,2 J. P. Blakeslee,2 R. J. Bouwens,2 T. J. Broadhurst,7 R. A. Brown,4 C. J. Burrows,4 E. S. Cheng,8 P. D. Feldman,2 M. Franx,9 D. A. Golimowski,2 L. Infante,10 R. A. Kimble,8 M. P. Lesser,11 W. J. McCann,2 F. Menanteau,2 G. R. Meurer,2 G. K. Miley,9 M. Postman,4 P. Rosati,12 M. Sirianni,2 Z. I. Tsvetanov,2 and W. Zheng2 Received 2002 December 23; accepted 2003 March 5 ABSTRACT The nearby and luminous QSO 3C 273 was imaged in 2002 July with the High Resolution Channel (HRC) of the Advanced Camera for Surveys (ACS) in coronagraphic mode in F475W (g), F606W (V ), and F814W (I ) as part of the Early Release Observations (ERO) program. After subtraction of the remaining PSF of the QSO, these images offer the most detailed view yet of the morphology and colors of the host galaxy of this QSO. We find that the central light distribution is elongated along the jet axis and its outer edge is delineated by an arc, centered on the jet at a radius of 2>6 from the QSO and bluer than the surrounding galaxy. This system is embedded in an extended galactic halo. Compared with early-type galaxies of similar redshifts and luminosities, the light distribution of 3C 273 is flatter in the core, likely from suppression by dust, but similar in the outer halo. The QSO is displaced from the isophotal center of the galaxy by 1>4. Previously known emission-line extensions are confirmed and new morphological features are identified, including a dramatic spiral-shaped plume, two faint filaments, a dust lane, and a knot along the jet axis. Part of the inner jet is unambiguously detected in all three bandpasses, and its morphology matches that of a MERLIN radio map. Different mechanisms that could explain the morphology of 3C 273 are considered, such as scattered QSO radiation, a face-on disk, and a merger event. Key words: galaxies: active — galaxies: nuclei — quasars: general — quasars: individual (3C 273) 1. INTRODUCTION of high-contrast imaging. As our observations show, mor- phological features in the host galaxy can be confidently In 2002 July the nearby, luminous QSO 3C 273 was described down to an unprecedentedly small radius of observed as part of the Early Release Observations (EROs) 1 3, even for a QSO as luminous as 3C 273. of the Advanced Camera for Surveys (ACS; Ford et al. > With every new advance in detector and telescope tech- 1998), installed in the Hubble Space Telescope (HST)in nology, 3C 273 has been one of the first QSOs observed 2002 March. This program was designed to demonstrate because of its relative proximity, high luminosity, and opti- the capabilities of the new instrument, and in particular for cal jet. Its host galaxy was first detected by Wyckoff et al. 3C 273, the on-orbit performance of the coronagraph. By (1980) and has since been the subject of numerous ground- suppressing the bright QSO, and with additional modeling with a template star, the ACS coronagraph represents a and space-based studies, including the landmark study of Bahcall, Kirhakos, & Schneider (1994, 1995a) and Bahcall major improvement over other HST instruments in the area et al. (1995b, 1997) with the Wide Field Planetary Camera 2 (WFPC2) of HST. It is therefore a natural choice for the 1 Based on observations made with the NASA/ESA Hubble Space first attempt at imaging with the ACS coronagraph; it may Telescope, obtained at the Space Telescope Science Institute, which is serve as a springboard for future coronagraphic studies of operated by the Association of Universities for Research in Astronomy, QSO hosts at higher redshifts and luminosities. Inc., under NASA contract NAS 5-26555. 2 Department of Physics and Astronomy, Johns Hopkins University, Here we describe our ERO observations of 3C 273. With 3400 North Charles Street, Baltimore, MD 21218; [email protected]. the superior PSF suppression capabilities and high spatial 3 UCO/Lick Observatory, University of California, Santa Cruz, 1156 resolution of the High Resolution Channel (HRC) and its High Street, Santa Cruz, CA 95064. coronagraph, we find that the morphology of the host gal- 4 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218. axy of 3C 273 is significantly more complex than had been 5 Department of Astronomy and Astrophysics, Pennsylvania State suggested in ground-based and HST/WFPC2 studies. University, 525 Davey Laboratory, University Park, PA 16802. Unlike most other HST QSO imaging programs, our images 6 Bartko Science and Technology, P.O. Box 670, Mead, CO 80542-0670. include three bandpasses acquired contemporaneously and 7 Racah Institute of Physics, Hebrew University, Jerusalem, Israel in identical instrument configuration. This multicolor 91904. 8 Laboratory for Astronomy and Solar Physics, NASA Goddard Space approach permits a better characterization of the emission Flight Center, Greenbelt, MD 20771. and absorption processes. At the redshift of 3C 273 9 00 Leiden Observatory, Postbus 9513, NL-2300 RA Leiden, Netherlands. (z = 0.1583), 1 corresponds to 2620 pc (for H0 =70km 10 Departmento de Astronomı´a y Astrofı´sica, Pontificia Universidad À1 À1 s Mpc and q0 = 0.1); the Galactic extinction toward this Cato´lica de Chile, Casilla 306, Santiago 22, Chile. 11 Steward Observatory, University of Arizona, Tucson, AZ 85721. system is E(BÀV ) 0.021 (Schlegel, Finkbeiner, & Davis 12 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 1998), and we use the Cardelli, Clayton, & Mathis (1989) D-85748 Garching, Germany. extinction law. 2964 IMAGING OF 3C 273 2965 2. OBSERVATIONS AND REDUCTION Before the subtraction (top), the most prominent feature is a scattered light bar, symmetric about the PSF. It is found to 3C 273 was observed as part of the ACS ERO pro- expand proportionately with wavelength—it might be due gram (SM3/ERO 8992, PI: H. C. Ford) with the High to large angle diffraction or a directional wave front error. Resolution Channel in coronagraphic mode. The ACS Residual diffraction rings are observed around the central coronagraph consists of two occulting spots and a pupil spot after the PSF subtraction (bottom). These are caused mask mounted on a mechanism that can be commanded by mismatches between the QSO and reference PSFs, specif- into the light path when needed. One spot has a diameter ically by differences in their positions relative to the center of 1>8 and is positioned near the field center, while the of the occulting spot. other spot is 3>0 in size and overlaps the end of an 0>8 Because we only have one PSF, we cannot do an exhaus- wide occulting finger. This finger is fixed at the CCD tive study of PSF subtraction effects. However, the residuals dewar window and is therefore imaged in both direct and caused by the normalization and shift errors are smaller and coronagraphic modes. A detailed description of the completely different in appearance than the galaxy features coronagraph is given in the ACS Instrument Handbook described below. The PSF residuals appear as rings around (Pavlovsky et al. 2002) and its on-orbit performance is the occulting spot and streaks radiating out from the QSO. described in Krist et al. (2002). The imaging device con- The galaxy’s inner features do not resemble these diffraction sists of a 1024 Â 1024 CCD manufactured by Scientific features. Imaging Technologies (SITe). The spatial scale at the The photometric zero points (ZEROPT) were calcu- detector was binned to 0>025 pixelÀ1, resulting in a nomi- lated with the BANDPAR task in IRAF/SYNPHOT, nal field of view of 2500 Â 2500. The HRC spatial resolu- giving 24.80 (F475W), 25.28 (F606W), and 24.54 (F814W) À À1 tion is 2 pixels (0>05). All the frames were acquired in in the AB magnitude system [mAB = À2.5 log (e s )+ normal operation and read out with amplifier C (read ZEROPTAB]. The transformation between the ACS and noise 4.7 eÀ) at a gain of 2 eÀ DNÀ1. Johnson filter systems will be available in the future The observations were made through the F475W, (Sirianni et al. 2002). For convenience, we will refer to F606W, and F814W filters on 2002 July 19–20 UT. Short F475W as g, F606W as V, and F814W as I. At the target acquisition frames (10 s through F502N) served to redshift of 3C 273, the [O ii]3727, H +[O iii], and translate the QSO under the central 1>8 diameter occulting H +[N ii] emission lines are shifted into the g, V, and I spot to an accuracy of 0.5–0.8 pixels (0>012–0>02). The jet bandpasses, respectively. was oriented away from the diffraction spikes and the charge bloom direction. Two orbits were dedicated to each bandpass (5130 s in F475W, 4750 s in F606W, and 4780 s in F814W) with a CR-SPLIT of two per orbit. A reference 3. RESULTS star, HD 105281, with colors similar to those of the QSO The PSF-subtracted images of 3C 273 in g, V, and was observed in one orbit in the same bandpasses for I are combined into a single image displayed in Figure accurate PSF subtraction.